Molecular drag pumping stages produce pumping action by momentum transfer from a fast-moving surface (moving at speed comparable to thermal speed of the molecules) directly to gas molecules. Generally, these pumping stages comprise a rotor and a stator cooperating with each other and defining a pumping channel therebetween. Collisions of gas molecules in the pumping channel with the rotor rotating at a very high speed cause gas in the channel to be pumped from the inlet to the outlet of the channel itself.
With reference to FIG. 1, between 1920-1930 Karl Manne Georg Siegbahn developed a molecular pumping device 10, wherein the pumping action is obtained through the cooperation of a rotor disk 20 having smooth surfaces integral with a rotating shaft 30 with a pair of stator bodies 40, 50, each facing a rotor disk surface and provided with a corresponding spiral-shaped groove 60 open towards the respective surface of the rotor disk and defining therewith a corresponding pumping channel.
The Siegbahn patent GB 332,879 discloses an arrangement of the above-mentioned kind. The gas to be pumped, entering through an inlet 70 at the outer periphery of each pumping groove, flows in both spiral channels in centripetal direction, i.e. from the outer periphery towards the center of the pumping grooves, as indicated by arrows CP. In this case two spiral pumping channels in parallel are to be considered; the gas flows in both channels in centripetal direction.
According to Siegbahn, in order to control the resistance of the gas pumped through the spiral channels 60, the cross-section area of these channels is reduced from the outer periphery of the stator bodies towards their center, in accordance with the reduction of the tangential speed of the disk, in the direction of the gas flow.
U.S. Pat. No. 6,394,747 (M. Hablanian) discloses a vacuum pump having reduced overall size and weight utilizing for this purposes a pair of Siegbahn-type pumping stages connected in series rather than in parallel.
According to U.S. Pat. No. 6,394,747 disclosure, a rotor disk having smooth surfaces is placed between a first stator disk and a second stator disk. Each stator disk is provided with a spiral groove open towards the respective surface of the rotor disk and defining therewith a corresponding pumping channel. At the beginning, the gas to be pumped flows between the first stator disk and the rotor disk in centrifugal direction, from the center to the outer periphery of the rotor disk, and then between the second stator disk and the rotor disk in centripetal direction, i.e. from the outer periphery to the center of the rotor disk.
The cross-section area of the groove defining the pumping channel in the first stator disk, where the gas flows in centrifugal direction, is reduced from the center to the outer periphery, while the cross-section area of the groove defining the channel in the second stator disk, where the gas flows in centripetal direction, is reduced from the outer periphery to the center. In this way the cross-section area of the grooves is always reduced in the direction of the flow and in this way, the U.S. Pat. No. 6,394,747 aims at optimizing both the pumping speed and the compression ratio.
In known Siegbahn-type pumping stage, having the above-mentioned geometric configuration generates the risk of internal compressions and successive re-expansions and corresponding power losses, especially in applications with important flow rates. Therefore, the main object of the present invention is to provide a spiral pumping stage for vacuum pump, which allows to overcome the above-mentioned drawback and to reduce power losses, especially when several stages are connected in series. This and other objects are achieved by a spiral pumping stage as claimed in the appended claims.